Vena caval filters can be utilized in conjunction with anti-coagulants and thrombolytic agents to prevent pulmonary embolism and other vascular diseases from occurring within the body. These devices are generally implanted within a vessel, such as the inferior vena cava, to capture dislodged blood clots (emboli) contained in the blood stream. If a blood clot forms in the deep veins of a lower extremity and dislodges, the blood clot may proceed up the vena cava into the heart and into the pulmonary arteries, where it may block and interrupt blood flow. Mortality is typically high in the event of pulmonary embolism.
Filtering devices that are placed in the vena cava have been available for a number of years. Various vena caval filters have been developed over the years, including the Mobin-Uddin umbrella filter, introduced in 1967 and discontinued in 1986. The Greenfield vena caval filter has been in wide use for a number of years and is known as the standard in vena caval filters.
To trap emboli, many conventional vena caval filters employ several independent filter legs that can be expanded within the vessel to form a substantially conical-shaped filtering profile within which emboli or clots can be collected. To prevent migration of the filter within the vessel, a hook, barb or other piercing or anchoring mechanisms on the filter leg can be used to secure the filter to the wall of the vena cava. For example, the Greenfield filter has multiple legs meeting at a central apex and has attachment hooks on the legs. Deployment of the Greenfield filter often occurs in a tilted fashion, which decreases clot capture ability of the filter. Moreover, the Greenfield filter is placed in the vessel in one direction that funnels clots to the apex of the filter and the center of the vessel. In addition, the attachment hooks on the legs of the Greenfield filter are also uni-directional and positioned for funneling clot to the apex of the filter. Thus, continued use of the Greenfield filter in the vessel may lead to accumulation of clots near the apex of the filter, and may further block and interrupt blood flow near the center of the vessel.
Furthermore, it should be noted that a percentage of patients only need a vena caval filter as protection from a pulmonary embolism for a short period of time. As such, leaving an implantable filter in place for an extended period of time may lead to complications, including inferior vena cava thrombosis, deep venous thrombosis, filter migration, and vena cava perforation. Therefore, in some circumstances, it may be desirable to remove the filter from the patient.
Removal of the filter from the vena cava, however, is met with certain hurdles. For example, some of these filters may not be easily removable from a patient due to fibrous in-growth into the filter. In particular, after deployment of a filter in a patient, proliferating intimal cells can start accumulating around the filter framework in contact with the wall of the vessel. After a length of time, such accumulation or in-growth can prevent removal of the filter without risk of trauma, requiring the filter to remain in the patient.
Another hurdle to removing a filter from the vena cava results from conventional vena caval filters becoming off-centered or tilted with respect to the hub of the filter as well as the longitudinal axis of the vessel within which the filter is situated. Removal of an off-centered or tilted filter can be difficult as the barbs or hooks securing the filter in place can dig further into the vessel walls and act to injure or damage the vessel during removal.
Accordingly, it would be desirable to have an effective vena caval filter that can be eliminated after the underlying condition has passed, while avoiding damaging the tissue of the vessel wall within which the filter is located.